No. 7. — The Origin of the Endocardium in Bony Fishes. By 



ARTHUK TeNNEY HOLBROOK.l 



The muscular walls of the heart in Vertebrates, it is now universally 

 agreed, come from the mesoderm, but concerning the origin of the endo- 

 thelial lining of that organ investigators differ widely. An idea of the 

 diversity of opinion in this matter may be formed from the enumeration 

 of the theories which Riicliert made in a paper read before the "Anato- 

 mische Gesellschaft " at Leipsic, in 1887. It is true the investigators 

 there mentioned conducted their studies on different Vertebrates, but the 

 disagreement is nevertheless significant. Balfour ('73) maintained that 

 the lining of the heart in the Chick came from splanchnic mesoderm ; 

 Eabl ('86), that in Amphibia it was derived from the endoderm ; while 

 Gotte ('74) asserted that both endoderm and mesoderm shared in its for- 

 mation. Blaschek ('85) believed that it was derived from the protover- 

 tebrte, and His that it came from parablastic cells which migrate from 

 the yolk ; whereas Eiickert himself maintained that the endothelial cells 

 arise from the endoderm of the ventral wall of the primitive gut, as well 

 as from the surrounding splanchnic layer of the mesnblast. 



The difference of opinion expressed upon this subject by various inves- 

 tigators, even in recent times, makes the topic one of continued interest. 



There are in Vertebrates, as is well known, two distinct types of heart 

 formation. The first type, that found in most of the Fishes and in Am- 

 phil)ians, results from tlie union of the two lateral plates of the mesoderm 

 in the median plane, the proximal edges of the plates forming the wall of 

 the heart, tlie space between the splanchnic and somatic parts of each 

 plate going to form the pericardial cavity. In this type, the formation 

 of the heart is subsequent to the completion of the primitive fore-gut. 



The second type of formation, that exhibited by Eeptiles, Birds, and 

 Mammals, is different, in that it takes place simultaneously with the 

 formation of the anterior end of the primitive gut, and consists in the 

 union in the median plane of two lateral blood-vessels, which are pre- 

 viously present in the mcsenchj-me. The simultaneous folding down- 



^ Contributions from the ZoriloRical Laboratory of tlie Museum of Comparative 

 Zotllogy, under the direction of E. L. Mark, No. XI.I. 

 VOL. XXV. — NO. 7. 



